Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.

Lightly flour your work surface and roll out the cold puff pastry to eliminate any creases.

Cut the pastry into 4x4-inch squares, yielding 6 to 8 pieces.

Spread a thin layer of Dijon mustard on half of the pastry squares and season lightly with salt and pepper.

Lay a slice of Gruyère cheese on the mustard, followed by a layer of ham, and add another slice of cheese on top.

Cover with another pastry square and seal the edges.

Beat an egg and brush the tops of each pastry stack with the egg wash.

Score a few decorative slits on the top layer of each stack for steam release.

Place prepared stacks onto the lined baking sheet and bake for 20 to 25 minutes until golden brown.

Let cool slightly on a wire rack and garnish with chopped herbs.